The present invention relates to signal converter, optical transmitter and optical fiber transmission system for transmitting a multichannel analog signal, a digital video signal or the like through an optical fiber.
In a suggested method for transmitting and distributing a multichannel video signal to users"" homes, the output light of a semiconductor laser is directly modulated with the multichannel video signal, and an optical signal resulting therefrom is transmitted through an optical fiber and directly detected by an optical receiver. An amplitude-modulated (AM) video signal transmission technique is currently in high demand, because this technique has excellent compatibility with existing CATV""s. However, in accordance with this technique, excellent carrier-to-noise ratio (CNR) and distortion characteristics are required. Accordingly, in employing this technique, the transmission distance and the number of branches of an optical fiber are adversely limited and the resistance of an optical fiber connector to reflection is disadvantageously low.
In order to solve these problems, a method for distributing a multichannel video signal using an AM/FM simultaneous converter of an optical heterodyne detection type is proposed. Such a method is described by K. Kikushima et al., xe2x80x9cSuper-wide-band Optical FM Modulation Scheme and its Application to Multichannel AM Video Transmission Systems, IEEE Photonics Technology Letters, pp. 839-841, 1996, for example.
Hereinafter, a conventional optical fiber transmission system of an AM/FM simultaneous conversion type will be described with reference to FIG. 1A through 1C.
As shown in FIG. 1A, an optical transmitter 1 of this optical fiber transmission system includes an AM/FM converter 2 and a semiconductor laser 3 for transmission. The optical transmitter 1 generates an optical signal. The intensity of the optical signal has been modulated with a microwave signal, the frequency of which has been modulated with an AM multichannel video signal. The optical signal output from the optical transmitter 1 is amplified by an optical fiber amplifier 4. The amplified signal is branched by an optical fiber coupler 5 into respective paths of an optical fiber 6, through which the optical signal is transmitted. The optical signal, which has been transmitted through the optical fiber 6, is received by an optical receiver 7. Specifically, an avalanche photodiode (APD) 8 of the optical receiver 7 receives the optical signal. The APD 8 converts the optical signal into a microwave signal, the frequency of which has been modulated with the AM multichannel video signal. And the microwave signal is demodulated by an FM demodulator 9 into the AM multichannel video signal.
FIG. 1B illustrates the internal configuration of the AM/FM converter 2. In the AM/FM converter 2, first, a semiconductor laser 10 is subjected to frequency modulation. Next, the output light of a local oscillator laser 11 is coupled with the output light of the semiconductor laser 10 at an optical coupler 12. Part of the coupled light is irradiated to a photodiode 14, which performs a heterodyne detection on the light so as to output a microwave signal having had the frequency modulated with the AM multichannel video signal. The carrier frequency of the microwave signal is equal to a beat frequency ("ugr"2xe2x88x92"ugr"1), which is the difference between the frequency "ugr"1 of the semiconductor laser 10 and the frequency "ugr"2 of the local oscillator laser 11.
The other part of the coupled light is irradiated to the other photodiode 13, which also performs a heterodyne detection on the light so as to output a microwave signal. The microwave signal is fed back to the semiconductor laser 10 through an auto frequency control (AFC) loop 15. This feedback loop can control the driving current of the semiconductor laser 10 and stabilize the carrier frequency. The AFC loop 15 includes an FM modulator 16 and an FM laser current controller 17.
Next, the semiconductor laser 3 for transmission (i.e., a distributed feedback (DFB) laser) is subjected to intensity modulation with the output signal of the AM/FM converter 2. As a result, the optical signal is output from the optical transmitter 1.
FIG. 1C illustrates the configuration of the FM demodulator 9 in the optical receiver 7. The FM demodulator 9 includes AND gates 18, 19, a delay line 20 and an amplifier 21 and demodulates the microwave signal output by the APD 8 into a multichannel video signal.
In accordance with such a transmission system, the minimum light-receiving level can be increased by about 10 dB and the reflective resistance of the optical fiber connector can be considerably improved as compared with a conventional AM transmission technique.
This conventional converter uses the AFC loop 15. However, if the temperature of the environment surrounding the semiconductor laser 10 changes, then the respective frequencies "ugr"1 and "ugr"2 of the semiconductor laser 10 and the local oscillator laser 11 also change. Accordingly, the intermediate frequency fIF (="ugr"2xe2x88x92"ugr"1) is greatly variable with the changing environmental temperature. FIG. 2A illustrates the spectra of the laser light emitted from the semiconductor laser 10 and the laser light emitted from the local oscillator laser 11. As for the laser light emitted from the semiconductor laser 10, both the spectrum of the laser light at the frequency "ugr"1 (having a relatively narrow distribution) and the spectrum of the AM multichannel video signal carried by the laser light (having a relatively broad distribution) are illustrated. FIG. 2B illustrates the spectrum of the carrier of the microwave signal at the intermediate frequency fIF (="ugr"2xe2x88x92"ugr"1) and the spectrum of the AM multichannel video signal carried by the carrier. As described above, as the intermediate frequency fIF (="ugr"2xe2x88x92"ugr"1) greatly changes with the changing environmental temperature, the spectrum of the intermediate frequency fIF shown in FIG. 2B has a broader width. FIG. 2C illustrates the intensity ratio of carrier to noise. As the width of the spectrum of the intermediate frequency fIF becomes broader, the intensity of the noise component increases relative to the intensity of the carrier. As a result, the CNR (carrier-to-noise ratio) decreases and the signal quality deteriorates.
In addition, in the conventional optical receiver 7, the optical signal needs to be received by the APD 8 operating in a broad band (e.g., 6 GHz) and the FM demodulator 9 needs to convert the optical signal into an electrical signal by using high-speed AND gates 18 and 19 that can operate at 6 GHz. In connecting high-speed devices such as these on multiple stages, since the frequency characteristics of the respective devices deviate from each other in terms of amplitude and group delay, such deviations are added to each other, thereby deteriorating noise and distortion characteristics. The broader band operation is advantageous in improving the noise characteristics. However, so long as electric devices such as AND gates are used, there is a limit on the high-speed operation. Since a band limitation is imposed, the deterioration of noise and distortion characteristics is inevitable.
In view of the above-described problems, the present invention was made to provide a signal converter exhibiting excellent CNR characteristics without using an AFC circuit, and provide an optical transmitter and an optical fiber transmission system showing high resistance to wavelength dispersion by using the same. Another object of the present invention is to provide a signal converter exhibiting excellent CNR characteristics and an optical transmitter and an optical fiber transmission system using the same.
A signal converter according to one aspect of the present invention receives a multichannel video signal and converts the video signal into a microwave signal, the phase of which has been modulated with the video signal. The signal converter includes: a laser light source; modulation means for modulating the phase of output light of the laser light source with the multichannel video signal and modulating the intensity of the output light with the microwave signal; and a light receiver for receiving the output light modulated by the modulation means and for outputting the microwave signal, the phase of which has been modulated with the multichannel video signal.
A signal converter according to another aspect of the present invention receives a multichannel video signal and converts the video signal into a microwave signal, the frequency of which has been modulated with the video signal. The signal converter includes: a laser light source; modulation means for modulating the frequency of output light of the laser light source with the multichannel video signal and modulating the intensity of the output light with the microwave signal; and a light receiver for receiving the output light modulated by the modulation means and for outputting the microwave signal, the frequency of which has been modulated with the multichannel video signal.
An optical transmitter according to one aspect of the present invention includes: the signal converter according to either aspect of the present invention; and a semiconductor laser for transmission. An output signal of the signal converter is superimposed with driving current of the semiconductor laser for transmission, thereby modulating the intensity of output light of the semiconductor laser with the output signal of the signal converter.
An optical transmitter according to another aspect of he present invention includes: the signal converter according to either aspect of the present invention; a semiconductor laser for transmission; and a light intensity modulator for transmission. The light intensity modulator for transmission modulates the intensity of output light of the semiconductor laser by using an output signal of the signal converter.
An optical fiber transmission system according to the present invention includes: the optical transmitter according to either aspect of the present invention; an optical fiber for transmitting an optical signal output from the optical transmitter; and a light receiver for converting the optical signal, transmitted through the optical fiber, into a multichannel video signal.
A multichanneled optical fiber transmission system according to the present invention includes: the optical transmitter according to either aspect of the present invention; an optical fiber amplifier for amplifying an optical signal output from the optical transmitter; an optical coupler for branching the amplified optical signal; an optical fiber for transmitting the branched optical signal; and a light receiver for converting the optical signal, transmitted through the optical fiber, into a multichannel video signal.
An optical signal converter according to the present invention includes: an optical output limiter for receiving an optical signal having had the intensity thereof modulated with a sub-carrier having had the frequency thereof modulated with a multichannel signal and for outputting light while the level of the waveform of the optical signal is at a predetermined level or higher, thereby generating a first optical pulse train; an optical delay circuit for delaying the first optical pulse train to generate a second optical pulse train; and an optical logic element for outputting a third optical pulse train having an optical pulse interval proportional to the multichannel signal based on the first and the second optical pulse trains.
An optical receiver according to the present invention includes: the optical signal converter of the present invention; and a light receiver for receiving the third optical pulse train, output from the optical signal converter, and converting the third optical pulse train into an electrical signal.
Another optical fiber transmission system according to the present invention includes: an optical transmitter for generating an optical signal, the intensity of which has been modulated with a sub-carrier having had the frequency thereof modulated with a multichannel signal; an optical fiber for transmission for transmitting the optical signal; and an optical receiver for receiving the optical signal through the optical fiber for transmission, and demodulating the multichannel signal from the optical signal. The optical receiver includes: an optical signal converter for outputting an optical pulse train based on the optical signal, the optical pulse interval of the train being proportional to the multichannel signal; and a photoelectric transducing section for receiving the optical pulse train and converting the train into an electrical signal.
Another multichanneled optical fiber transmission system according to the present invention includes: an optical transmitter for generating an optical signal, the intensity of which has been modulated with a sub-carrier having had the frequency thereof modulated with a multichannel signal; an optical fiber amplifier for amplifying the optical signal; an optical coupler for branching the optical signal into a plurality of luminous fluxes; an optical fiber for transmission for transmitting the branched optical signal; and an optical receiver for receiving the optical signal through the optical fiber for transmission, and demodulating the multichannel signal from the optical signal. The optical receiver includes: an optical signal converter for outputting an optical pulse train based on the optical signal, the optical pulse interval of the optical pulse train being proportional to the multichannel signal; and a photoelectric transducing section for receiving the optical pulse train and converting the train into an electrical signal.